A large number of dead harbor porpoises have been washing up on beaches in San Mateo County and elsewhere in the Bay Area this summer, and marine mammal experts are at a loss to explain what could be killing them.

A total of 24 harbor porpoises — 12 males, 9 females, and three still in utero — have been discovered by beachcombers since early June, according to the Marine Mammal Center in Sausalito. More carcasses are likely to show up in August.

"It's the tip of the iceberg. These are open ocean animals. For every one we find dead there are probably many others that are out there," said Mary Jane Schramm, spokeswoman for the Gulf of the Farallones National Marine Sanctuary.

Harbor porpoises are not commonly tracked by the state and so little is known about where they feed or mate. Over time, state records show the mammals have a tendency to die during the summertime calving season, but they don't know why. The number of dead porpoises statewide peaked at 49 in 2005; the 24 collected so far this summer were found just in Northern California, between Marin and San Luis Obispo counties.

The five porpoises that washed up in San Mateo County were found in Pacifica and along Thornton Beach in Daly City.

One unusual factor is that the harbor porpoises were young and old, male and female, said Jim Oswald, a spokesman for the Marine Mammal Center.

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Only one porpoise had obvious signs of trauma indicating that it had been hit by a boat or some other heavy object. Preliminary causes of death — such as pneumonia, asphyxiation and maternal separation — have been determined for a handful of other porpoises following necropsies, but these cases have had little in common.

Veterinarians ordered toxicology tests for the remaining harbor porpoises. They will be looking for one particular potential cause of death.

"There's a possibility that it's domoic acid poisoning," said Oswald, referring to the toxin produced by a particular kind of algae that, if ingested, causes severe brain damage or death in marine mammals. Last year, domoic acid killed 10 percent of the sea lions necropsied by the Marine Mammal Center, the main destination for dead and stranded Bay Area mammals.

Demoic acid is just a working hypothesis for now, since the evidence for it is slim. The kind of harmful algal blooms that get out of control tend to occur in Southern California, where waters are warmer, and research done just this week by scientists with the Gulf of the Farallones National Marine Sanctuary found only small concentrations of it in the Bay, according to Schramm.

Previous tests have detected Pseudo-nitzschia, the type of algae that produces domoic acid, out in the ocean within the sanctuary's boundaries, but not enough to give the California Department of Public Health, which monitors it, any great cause for alarm.

Still, Schramm sees some possible evidence of domoic acid poisoning in the fact that she was told that at least two of the mother porpoises that died while pregnant were carrying fetuses that had already died by the time the mother succumbed.

"It could be that the acid bio-accumulated in the fetus. If it's something that the mother ingested and passed through the placental barrier, it could be something that she passed on to her fetus," said Schramm.

National Marine Fisheries Service wildlife biologist Joe Cordero has studied marine mammal mortality for years. He recognizes the trend in harbor porpoise deaths, but says the numbers don't point to any cause of death that would be easy to characterize — not even in 2004, when almost 50 of them died.

"There was no smoking gun, so there was nothing to solve. If it's not a health-related issue, there's nothing you can do as a wildlife health manager," said Cordero. "There's a whole lot of questions, there not a whole lot of answers."